1. Field of the Invention
The invention relates generally to optical crystals for transmitting infrared, visible, and ultraviolet (UV) radiation. In particular, the invention relates to a method and an apparatus for evaluating the quality of an optical crystal.
2. Background Art
The trend in microelectronics is towards increasingly smaller scale. This is made possible by lithography techniques that can print fine patterns, allowing the development of integrated circuits that pack increasing density in the same area. The remarkable progress in lithography has been made possible by development in many fronts, including improved lens quality, increase in numerical aperture, improved resist processes, and the use of increasingly shorter exposure wavelengths. Currently, advanced microlithography systems use deep-UV radiation having a wavelength of 248 nm (KrF laser) to print 0.25-xcexcm features. New microlithography systems using 193-nm radiation (ArF laser) and 157-nm radiation (F2 laser) are actively under development and are expected to produce even smaller features. However, finding suitable lens materials for these shorter wavelengths poses a challenge. At present, high-purity fused silica and fluoride crystals are the practical choices for transmitting UV radiation. Development of 157-nm lithography requires fluoride crystals because the transmission properties of fused silica drops off significantly below 180 nm.
The performance required from fluoride crystal materials is not fully understood, yet the industry has to be served with components and materials that function properly. One of the several indicators that can be used to qualify a crystal material is fluorescence. Some laser manufacturers have found that the way a piece of fluoride crystal, especially calcium fluoride crystal, fluoresces may be linked to the way it performs in their excimer lasers. Some of these laser manufacturers have specified that crystal materials used in their lasers should be free of any visible fluorescence when exposed to deep-UV radiation (below 300 nm wavelength). It is not clear how laser performance is affected if the crystal material fluoresces. However, what is clear is that internal heating of the crystal material and durability of the material upon exposure to radiation at these wavelengths are important factors in lithography applications. For example, heating of the crystal material can change the refractive index of the material, resulting in phase shift errors imposed on the wavefront of a beam transmitted through the material. Optical lithography systems have very little tolerance for phase errors.
It should be noted that heating of the crystal material by passing a laser beam through it can have several mechanisms, only some of which may be accompanied by fluorescence. Thus, the aim is not to make a judgment about total losses by looking at fluorescence. Furthermore, it is not obvious that there is a connection between the fluorescence spectrum of the crystal and how constant the laser transmission remains as the laser beam continues to be passed into the crystal. Hence, finding the connection between fluorescence of the crystal and anything that really affects the performance of the crystal in the presence of the laser beam is still active research guided by measured correlated data. Notwithstanding the fact that the relationship between fluorescence of the crystal material and the laser performance is unclear, it would still be useful to have a mechanism for grading optical crystals based on fluorescence. Such grading may be one of the factors used in determining the suitability of the optical crystal for lithography applications or laser applications in general.
In one aspect, the invention relates to a method for evaluating the quality of an optical material. The method comprises obtaining a fluorescence spectrum of the optical material, obtaining a fluorescence spectrum of a reference material having desired performance in a target application, and determining whether a shape of the spectrum of the optical material is similar to a shape of the spectrum of the reference material. If the shape of the spectrum of the optical material is similar to the shape of the spectrum of the reference material, the method includes indicating that the optical material is suitable for the target application; otherwise, the method includes indicating that the optical material is unsuitable for the target application.
In another aspect, the invention relates to an apparatus for evaluating the quality of an optical material. The apparatus comprises a source which emits an excitation light and a plurality of optical elements which focus the excitation light on the optical material. The apparatus further includes a spectrometer which detects fluorescence light emitted from the optical material. A line of sight of the spectrometer is oriented at an angle with respect to a primary axis of the excitation light transmitted through the optical material. The apparatus further includes a processor which runs a process that compares the fluorescence data from the spectrometer to a reference fluorescence spectrum and determines the quality of the optical material based on the comparison.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.